A self-tightening chuck for a hammer drill normally has a chuck body centered on an axis and adapted to be rotated there-about, a plurality of jaws radially displaceable on the body, and an adjustment sleeve rotatable on the body, formed with radially inwardly directed teeth, and engaged with the jaws such that rotation of the adjustment sleeve on the body in a forward direction displaces the jaws radially inward on the body and opposite rotation in a reverse direction displaces the jaws radially outward. A locking body radially displaceable on the chuck body is formed with teeth meshable with the tightening-ring teeth and a locking spring braced radially against the locking body urges the locking-body teeth into mesh with the adjustment-sleeve teeth. The locking-body and adjustment-sleeve teeth are angled such that they lock together and rotationally couple the locking body to the adjustment sleeve on forward rotation of the chuck body relative to the adjustment sleeve but slide past each other on forward rotation of the adjustment sleeve relative to the chuck body. A locking ring angularly displaceable on the chuck body has a formation engageable with the body and is displaceable between an unlocked position displacing the locking body radially inward out of engagement with the adjustment sleeve and a locked position leaving the locking body pressed radially by its locking spring against the adjustment sleeve.
As described in my U.S. Pat. No. 4,272,087 such a chuck is actuated by a standard geared chuck key. Here the locking body is a chordally movable pin whose flat end fits between the ad-justment-body teeth, which in this situation are shaped as isoscelese right triangles. The end of the locking pin is perpendicular to its longitudinal axis and sits flatly against the trailing flanks of the adjustment-sleeve teeth, that is with the pin axis extending at 45.degree. to a radius through the chuck axis. The locking pin is angled such that when the adjustment sleeve is rotated forward relative to the chuck body the pin ratchets on the adjustment-sleeve teeth, but on opposite rotation the teeth lock on each other and loosening of the chuck is prevented.
With such a chuck the user must twist the locking ring to the unlocked position so the adjustment sleeve can be rotated backward to release the bit. Then after a new bit is chucked, the locking ring must again be reset in the locked position to prevent the chuck from getting loose from vibration. Furthermore if during use the chuck loosens, for instance because the jaws bite into the shank of the bit, the user must manually retighten the chuck.